The choice is Marty and Brian in some combination.
Marty has proven 4 different times that he knows how to change a culture of losing, and he can do it quickly. Brian brings a special gift of being able to develop young QBs like Brees, Rivers and Sanchez. If he knows how to develop them, he probably has an eye on what makes a good QB as well, which the Bills desperately need.
Both of them have a good working relationship with Buddy.
The opportunity to coach with his son and long time friend set up the unique situation that will bring him back for a short term until his son is ready to take over.